Various processes for producing color positives on paper or film are know. In the so-called direct color positive process, the color positive is produced as a direct reproduction from a positive original, that is, using only a single sheet of positive material, while in the so-called two-sheet diffusion transfer process, an intermediate negative is needed, which is then placed in contact with a positive transfer material and activated for diffusion in an activator bath. In this process, dictated by the emulsion used, a contact screen of the above type, also known as a contrast control foil, is required in order to lend definition to the negative material.
In the known contact screens, the screen elements, which as a rule are arranged in an elliptical, checkerboard or grid pattern, have a pyramidal course in terms of their density, i.e., the measure of their degree of optical opacity, from edge to edge; in other words, they have their greatest density in the middle or center, and the density then decreases toward the edges of a given screen element and merges more or less continuously with the adjoining transparent element.
Since the color positives (reproductions) produced by the use of such generally neutral-gray contact screens have a reddish tinge in the bright areas, blue-green colored contact screens have already been used as a way to avoid this tinge. However, it is expensive to produce such contact screens with a standardized blue-green coloring. Also, they cannot reliably prevent a tinge in the positive, because the photosensitive material itself cannot be produced in a sufficiently standardized way and hence exhibits fluctuations in the sensitizing of the color layers. While fluctuations in the sensitizing of the negative material can be compensated for when neutral gray contact screens are used, this can be done only with difficulty in the case of colored contact screens.
The essential disadvantage common to both the known, neutral gray or colored, contact screens is that the screen structure is reproduced on the positive. This reproduction of the screen structure is not perceptible to the eye on the reproduced positive, but if this positive is used as an intermediate original for making a screened reproduction for use in printing (original reproduction copy), the superimposing of the contact screen structure that is reproduced on the intermediate original and of the screen used to produce the original reproduction copy leads to a visible moire pattern on the reproduction, which accordingly reappears in the printed copy. A further disadvantage of the known contact screens is that insufficient edge definition is attainable on the positive that is produced; this is very troublesome, particularly when reproducing written materials and fine lines.